Europe is stepping up its preparations for the launch of a space craft designed to chase and land on a comet.

The Rosetta mission was scrubbed at the last minute in January because of problems with the Ariane rocket system that would have launched it from Earth.

The orbiter will drop a lander on a comet

It will now lift off in February 2004, after a delay of more than a year.

Project engineers are about to enter the final launch campaign after a summer spent modifying the space probe.

Scientists have had to choose a new target comet - a ball of ice and rock known as Churyumov-Gerasimenko.

The new destination has posed some engineering challenges for the European Space Agency (Esa), not least because the comet is three times larger than Wirtanen, its original quarry.

When Rosetta reaches the comet, it will drop a 100-kilogram lander, the size of a washing machine, which will descend at walking speed and harpoon itself to the comet's surface.

Ariane is in good shape and I think the space craft is ready to go

John Ellwood, Project Manager, European Space Agency

Engineers were concerned that the lander, designed to touch down on a much smaller body, could travel too fast and topple over because of the comet's stronger gravitational pull.

To overcome this, they have fitted a mechanical device to stop it tilting, in addition to the lander's existing three legs which will unfold and absorb the energy of the impact.

"We've had to look at all the mission elements and make sure this new mission can be satisfied by the space craft that was ready to be launched in January 2003," said John Ellwood, project manager at Esa.

"I think we're very confident we've looked at all the mission aspects - Ariane is in good shape and I think the space craft is ready to go."

Rocket modification

Rosetta is scheduled for launch on 26 February 2004 on an Ariane-5 G+ rocket from Europe's spaceport in Kourou, French Guiana.

The Rosetta orbiter undergoing thermal tests

The booster has a similar configuration to the one used to shoot Europe's Smart 1 mission at the Moon in September.

Rosetta, which has been stored in a facility in Kourou for the past nine months, will now go through the final testing programme.

Its communications system will be checked out; giant solar arrays - the width of a football pitch when deployed - will be re-fitted; and, in January, fuel will be pumped on board.

Stardust

The New Year promises to be an exciting time for cometary missions. The US space agency (Nasa) will direct its Stardust space craft to fly by Comet Wild 2 at the beginning of January.

The probe will capture up-close pictures of the icy heart of a comet for only the third time ever. For the first time in history, it will then try to snatch a sample of dust due for return to Earth in 2006.

Approaching at a speed of six kilometres per second (14,000 miles per hour), Stardust will try to capture the tiny particles that make up the comet's tail.

Dr Mark Burchell, of the University of Kent in southern England, is a collaborator on the mission.

"We will have freshly emitted dust grains from a comet and we know which comet they came from", he told BBC News Online, "we'll know what that comet is made of and we'll know what the early Solar System was made of."